1. Field of the Invention
The present invention generally relates to an intravenous infusion of medical solutions to a patient and, more particularly, to a method of sampling an output signal generated from a bubble detector in a peristaltic I.V. infusion pump apparatus.
2. Description of the Prior Art
The I.V. infusion device now in wide use makes use of a bubble detector for detecting the presence of bubbles in a medical solution then flowing through an I.V. line leading to a patient, By sampling output signals emerging from the bubble detector, the size of a bubble present in the medical solution can be determined.
The I.V. infusion device generally includes a peristaltic infusion pump for the intravenous administration of a medical solution to a patient. An example of the prior art peristaltic infusion pump is generally shown by 60 in FIG. 7. As shown in FIG. 7, the prior art peristaltic infusion pump 60 comprises upper and lower support arms Ca and Cb spaced apart from each other to define a finger chamber therebetween, a drive shaft 62 rotatably supported by the upper and lower support arms Ca and Cb by means of suitable bearings, a plurality of, for example, eight, cam lobes 63a to 63h mounted eccentrically on the drive shaft 62 for rotation together therewith, and fingers 64a to 64h equal in number to the cam lobes 63a and 63h housed within the finger chamber of the casing C and adapted to be successively driven in a direction perpendicular to the drive shaft 82 during the rotation of the drive shaft 62.
Each fingers 64a to 64h has an aperture defined therein so as to receive a corresponding cam lobe 63a to 63h therein. The cam lobes 63a to 63b are rigidly mounted on the drive shaft 62 in a helical pattern along the drive shaft 62 so that, during each complete rotation of the drive shaft 62 in one direction driven by a suitable drive motor (not shown), the cam lobes 63a to 63h will successively drive the respective fingers 64a to 64b in a direction perpendicular to the drive shaft 62 with the fingers 64a to 64h consequently producing a peristaltic action to that portion of an elastic infusion tubing 61 which extends so as to traverse the finger chamber in a direction generally parallel to the drive shaft 62 and which is positioned between a backup plate 68 and respective finger tips of the fingers 64a to 64h. It is to be noted that the infusion tubing 61 has an upstream end communicated with a known source of medical solution to be intraveously infused and a downstream end communicated with a needle or catheter and then into a vascular system, for example, a vein, of a patient.
In general, the illustrated peristaltic infusion pump is so designed that, during each complete rotation of the drive shaft 62, the cam lobes 63a to 63h successively drive the associated fingers 64a to 64h to cause the latter to undergo a linear peristaltic motion with the finger tips consequently sequentially squeezing that portion of the infusion tubing 61 while producing at least one moving zone of occlusion along said infusion tubing, thereby to transport the medical solution successively towards the downstream end of the infusion tubing 61 and then towards the vascular system of the patient.
As is well known to those skilled in the art, the flow of the medical solution through that portion of the infusion tubing 61 takes place only during a period from the timing at which the finger tip of the uppermost or most upstream finger 64a squeezes the infusion tubing 61 in cooperation with the backup plate 68 to the timing at which the finger tip of the lowermost or most downstream finger 64h subsequently squeezes the infusion tubing 61 in cooperation with the backup plate 68.
On the other hand, no flow of the medical solution through that portion of the infusion tubing 61 takes place during a period from the timing at which the finger tip of the lowermost or most downstream finger 64h squeezes the infusion tubing 61 in cooperation with the backup plate 68 to the timing at which the finger tip of the uppermost or most upstream finger 64a subsequently squeezes the infusion tubing 61 in cooperation with the backup plate 68. This period is well known as a deadband, and this deadband occurs as a result of a difference in pressure between respective portions of the infusion tubing 61 upstream and downstream of the finger tip of the most downstream finger 64h then squeezing the infusion tubing 61.
It is to be noted that, for the purpose of the present invention, while the second mentioned period during which no flow of the medical solution take place is known as the deadband, the first mentioned period during which the flow of the medical solution takes place is referred to as a liveband.
According to the conventionally practiced sampling technique, the physical inherence of the deadband and the liveband in the sophisticated peristaltic infusion pump is not taken into consideration, and an output signal emerging from a bubble detector is generally sampled at a predetermined uniform interval by the use of a timer over the entire sampling period. This means that, even during the deadband period, the output signal emerging from the bubble detector is sampled for the detection of the presence or absence of a bubble in the infusion tubing.
Considering that no flow of the medical solution through the infusion tubing take place during the deadband, no change take place in the output of the bubble detector and, therefore, the sampling of the output signal emerging from the bubble detector has no significance and is nothing other than the futility. Also, the sampling of the detector output during the deadband period tends to adversely affect the algorith, used to detect the presence of the bubble in the medical solution within the infusion tubing, to such an extent as to result in a variation in size of bubbles detected by the bubble detector. By way of example, assuming that the algorith is employed of a kind designed to detect bubbles of a predetermined size in reference to the length of time over which the bubble intercepts the bubble detector, and if the deadband starts at the moment the bubble comes to a position intercepting the bubble detector as shown in FIG. 5(a), the bubble remains intercepting the bubble detector throughout the deadband period. Therefore, if the sampling continues even during the deadband, the detector output indicative of the presence of the bubble will have a duration longer than that generated from the bubble detector during the flow of the medical solution taking place, that is, during the liveband, and therefore the bubble detector will be apt to detect bubbles of a size smaller than the bubble size desired to be detected.